This invention relates to optical wave propagation systems and devices utilizing electro-optical devices, and more particularly to grating assisted devices for filtering, coupling and other functions.
Communication systems now increasingly employ optical waveguides (optical fibers) which, because of their high speed, low attenuation and wide bandwidth characteristics, can be used for carrying data, video and voice signals concurrently. An important extension of these communication systems is the use of wavelength division multiplexing, by which a given wavelength band is segmented into separate wavelengths so that multiple traffic can be carried on a single installed line. This extension requires the use of multiplexers and demultiplexers which are capable of dividing the band into given multiples (such as 4, 8, or 16 different wavelengths) which are separate but closely spaced. Adding individual wavelengths to a wideband signal, and extracting a given wavelength from a multi-wavelength signal, require wavelength selective couplers, and this has led to the development of a number of add/drop filters, the common terminology now used for devices of this type.
Since wavelength selectivity is inherent in a Bragg grating, workers in the art have devised a number of grating-assisted devices for adding or extracting a given wavelength with respect to a multi-wavelength signal. Typical optical fibers propagate waves by the use of the light confining and guiding properties of a central core and a surrounding cladding of a lower index of refraction. The wave energy is principally propagated in the core, and a number of add/drop filters or couplers have been developed using Bragg gratings in the core region of one of a pair of parallel, closely adjacent or touching fibers. The coupling region is commonly termed xe2x80x9cevanescentxe2x80x9d in that a signal propagated along one fiber couples over into the other, as an inherent function of the design. The wavelength selectivity is established by the embedded grating, which provides forward or backward transmission of the selected wavelength, depending upon chosen grating characteristics. For modern communication systems, however, this approach has a number of functional and operative limitations, pertaining to such factors as spectral selectivity, signal-to-noise ratio, grating strength, temperature instability and polarization sensitivity.
The applications referenced above are based upon a novel theoretical concept and practical implementation. A narrow waist region of two fused dissimilar fibers is defined between pairs of tapered coupling sections at each end. At the waist, he merged fibers are formed by elongation of an optical fiber precursor of generally conventional size and are so diametrically small that the central core effectively vanishes. The wave energy is transferred through the merged fiber region in two spatially overlapping, orthogonal modes. Since the propagating energies of the modes overlap, the coupling is essentially non-evanescent in except the presence of a coupling mechanism such as a diffraction grating. For example, a reflective grating written in the waist region redirects only a selected wavelength of an input signal at the input port to the drop port, while all other wavelengths propagate through the waist section without reflection to the throughput port. This reflection grating thus couples light between two optical modes in a non-evanescent manner. Numerous advantages derive from this concept and configuration, but the realization of its full potential is dependent upon other developmental factors.
For example, modern applications require that any add/drop filter based upon this concept be very efficient at routing channels, have a strong grating which can be selectively and precisely placed at or adjusted to a specific wavelength and yet have a limited bandwidth, be temperature insensitive, compact, low cost, and not subject to spurious reflections or noise in the chosen wavelength band. Achieving high drop efficiency and low polarization dependence are particularly important. The problems of achieving these operative properties while at the same time providing a repeatably producible unit of very small size and high sensitivity have required much further innovation.
In accordance with the invention, the optical properties and performance of a grating assisted asymmetric fused coupler are highly dependent on the physical characteristics of the coupler waist. Polarization insensitivity of the drop wavelength can be achieved, for example, by controlling the shape during elongation or by applying a permanent twist to the coupler waist after the grating exposure. Furthermore, the small diameter waist renders the coupler sensitive to diameter non-uniformities but it is shown that these dimensional variations can be compensated by laser trimming or by impressing a compensated index of refraction grating. Further, the strength of the grating can be dramatically increased by in-diffusing a photosensitizing gas during the grating writing process. For improved spectral characteristics the grating is apodized and unchirped by being written with concurrent grating modulated (a.c.) and uniform (d.c.) intensity UV beams. Size and other characteristics of the waist region are selected such that the drop wavelength of the coupler is adequately separated from the backreflection wavelength and the latter wavelength lies outside the frequency band of interest.
A small coupler having these properties and wavelength adjustability as well is enclosed within a prepackage structure which enables optical access to the coupler waist for grating writing. An elongated structure consisting of materials having different thermal coefficients of expansion is arranged to compensate the temperature dependence of the drop wavelength. Moreover, the structure provides fine tuning so that the drop wavelength is precisely adjusted and subsequently maintained throughout the desired operating temperature range.
Methods and apparatus for writing high strength, precisely defined gratings in very narrow optical fiber structures utilize precision mechanisms and optical subsystems as well as unique processing steps. A merged waist region forming a non-evanescent coupler is first formed in a manner rendering the coupler polarization insensitive. Shape control of a precise nature is achieved by analyzing polarization response during elongation as a transversing flame or CO2 laser beam softens the fibers, and using the measured polarization response to control the heat source in a manner that minimizes polarization sensitivity. Because of the minute diameter of the elongated waist region very small width variations can affect grating uniformity, but this is compensated by varying the background index of refraction. The waist region is rendered photosensitive to impinging UV light by performing all writing operations within a pressurized hydrogen or deuterium environment which assures in-diffusion of photosensitizing gas and replenishes interior gases as the reaction proceeds, maximizing grating strength and uniformity.
To compensate index of refraction variations along the length of the waist region, a test grating is written along the waist and the wavelength response is measured at progressive locations along the waist. In accordance with the readings the background index is varied locally so that a net equalized value exists along the region in which the grating is to be written. During writing the laser beam spot, which is large relative to the waist, is positioned accurately by use of a servo system which adjusts beam position in response to the light transmitted through the fibers. An apodized grating is written in the waist by dividing a scanning laser beam, in accordance with the apodization function, into a periodically varying (i.e. a.c.) beam, and a d.c. beam. These are concurrently directed onto the waist region to create an apodized pattern.